1. FIELD OF THE INVENTION
This invention relates to a driver circuit for a liquid crystal display that can be employed in a liquid crystal television receiver or the like.
2. DESCRIPTION OF THE PRIOR ART
In recent years, pocket-sized liquid crystal television receivers have been available. In such a liquid crystal television receiver, each of switching elements comprising FET's provided at respective elemental liquid crystal cells arranged in a matrix shape on a liquid crystal panel is supplied with an input image signal voltage and a switching signal and is connected with one electrode of each liquid crystal cell. A common electrode commonly connected with the opposite electrode of each liquid crystal cell is supplied with a common voltage. Since the liquid crystal panel must be A.C. driven, the input signal voltage and common voltage are inverted in their polarity every one field.
Referring now to the drawings, an example of the prior art driver circuit for liquid crystal displays will be explained below.
In FIG. 1 showing a matrix type liquid crystal display, a liquid crystal cell 1, storage capacitor 2 and field effect transistor (FET) 3 that serves as a switching element constitute a liquid crystal display element for displaying each picture element (pixel). An X-electrode 4 is supplied with a switching signal and a Y-electrode 5 is supplied with an image signal. A common electrode 6 provided on an opposite substrate is supplied with a common voltage.
In FIG. 2 showing one display element in detail, 1 to 6 denote like parts in FIGS. 1 and 7, 8 and 9 denote capacitances C.sub.GS, C.sub.GD and C.sub.DS among the electrodes of the FET, respectively. Y-electrode 5 is supplied with an image signal that is inverted in its polarity every one field as shown by 10 in FIG. 3 and sampled by each switching element for each pixel. Common electrode 6 is supplied with a common voltage that is inverted in its polarity every one field as shown by 11 in FIG. 3. The image signal voltage is applied to one electrode of the liquid crystal cell 1 when FET 3 is turned on by the switching signal applied to X-electrode 4. This switching signal turns on FET 3 during 1H (H denotes a horizontal scanning period: 63.5 .mu.sec. and turns it off during the remaining about one-field period (16.7 m sec.). Storage capacitor 2 holds during the "off" period a charge corresponding to the image signal voltage applied during the "on" period. The drive voltage applied across the liquid crystal cell 1 is inverted in its polarity during the subsequent one field, for A.C. drive of the liquid crystal panel.
FIG. 4 shows a circuit for inverting the image signal and the common voltage. In FIG. 4, 12 denotes an PG,4 input terminal of a switching signal V.sub.T that is changed into a high/low level every one field. This V.sub.T signal is employed to switch, every one field, inverter circuits 14 and 15 to alternately derive, the image signal applied to an image input terminal 13 and its polarity-inverted image signal, and to alternately derive a common voltage V.sub.1 obtained by dividing a power voltage V.sub.cc by resistances R.sub.1, R.sub.2 and R.sub.3 and its polarity-inverted voltage V.sub.2. Namely, the image signal is inverted in its polarity every one field and sent to an image output terminal 16. This polarity-inverted image signal is applied to Y-electrode 5 of FIG. 2 through a Y-driver. The V.sub.1 and V.sub.2 voltages, polarity-inverted every one field, are sent to a common voltage terminal 17 and applied to common electrode 6 of FIG. 2.
The above mentioned prior art arrangement suffers from the following disadvantages.
Since, as shown in FIG. 2, inter-electrode capacitances C.sub.GS 7, C.sub.GD 8 and C.sub.SD 9 exist among the electrodes of FET 3, and also the capacitance of storage capacitor 2 may vary because of the fabrication process of the liquid crystal panel, the image signal voltage and the common voltage applied to an electrode of a liquid crystal cell may not be correctly related. More specifically, although the polarity-inverted voltage must be applied across a liquid crystal cell with a predetermined voltage difference every one field, the level of image signal voltage 10 may vary at one electrode of the liquid crystal cell because of the above variation as shown, for example, by the one-dotted chain line in FIG. 3. Thus, the applied voltage may be partially inverted as shown by the dotted arrow in FIG. 3 or the difference between the image signal voltage and the common voltage (i.e. the amplitude of the voltage applied across the liquid crystal cell) may fluctuate among the respective fields.